The present invention relates generally to welding systems, and particularly to a technique for controlling power applied to a welding arc.
Various techniques are known for welding two or more metal pieces to one another. These techniques typically rely upon application of electrical current through a welding torch, and thereby through an electrode and work piece to heat the work piece and the electrode. In arc welding, an arc is established between the electrode and the work piece, with the arc generating sufficiently high temperatures to melt the work piece, and in certain types of welding the electrode as well. In a type of welding commonly known as “stick” welding, for example, a stick electrode is held in a clamp-like handle and the tip of the electrode is brought into close proximity with a point on a work piece where a weld bead is to be applied. The stick electrode is brought sufficiently close to the work piece to initiate an arc, and may then be drawn back from the work piece to regulate heating of the work piece and of the electrode, and progression of the weld bead. Various types of stick electrodes are known and are currently in use for different types of applications and metals to be welded. These electrodes often have quite different properties, and perform differently in the welding operation.
In typical welding applications, the supply of electrical power to the electrode is regulated, and in many cases may be at least somewhat controlled by the welding operator. Typical controls include constant voltage control and constant current control. Other control schemes may be envisaged, with hybrid schemes being relatively common in the art. Power supplies for stick welding of the type described above are often controlled in a constant current scheme, whereby the current applied to the electrode, and that passes through the arc and work piece, is maintained generally constant over a range of voltages. During the welding operation, an operator typically strikes an arc by touching the electrode tip to the work piece, and then increases the distance between the electrode tip and the work piece during welding. By controlling the distance of the electrode tip from the work piece, the welding operator can regulate the heating of the work piece and the electrode and generally control the weld. To cool the weld, for example, the operator may draw the electrode from the work piece, elongating the arc in a movement commonly referred to as “whipping”. Conversely, the operator may press the electrode close to the work piece to penetrate move deeply into its surface in a movement referred to as “digging”.
Generally, in a constant current control regime, as the arc length decreases, the voltage applied to maintain the arc also decreases. However, in these operations if the arc becomes sufficiently short, it can be extinguished by the decrease in voltage. To avoid such consequences, conventional constant current power supplies for welding often include a “dig” control which effectively alters the slope of a voltage/current line (increasing the current applied beyond the constant current level) beginning at a pre-determined minimum voltage. Regardless of the dig setting, however, the voltage at which the increase in current begins is identical in such systems. Such control does not take into account differences in the performance of various electrodes. Indeed, while this voltage at which the increase in current begins may work well for certain electrodes, it will typically work much less well for others.
Another feature of conventional constant current controls for welding is a relatively sloppy control of the voltage/current input when the arc becomes longer than a particular length (i.e., above a certain voltage). In general, a welding operator may back the electrode tip significantly off of the work piece, particularly with certain electrodes, to quickly reduce power input. The relatively lax control of the voltage/current waveform, however, makes this operation relatively unpredictable. That is, the operator may not be able accurately to predict how the arc will react as the electrode is drawn farther back from the work piece.
There is a need, therefore, for improved techniques for regulation of constant current welding controls and power supplies. There is a particular need for improved dig control that allows welding operators to adjust system performance to conform to desired operating conditions and particularly to different types of electrodes. There is also a need for an improved constant current control scheme that allows the welding operator to extend the arc significantly and have the constant current control regime react in a predictable and rapid manner, preferably maintaining a constant current regime for a relatively predictable arc length based upon the user input.